In a low, cool voice, Boggess Steve Davis, 20, told a crowded Alexandria courtroom yesterday that he shot and killed a government economist last January and wounded his wife in a robbery to get money for Davis to take his 17-year-old girlfriend to the movies.

"I was going to call a relative for the money or make a hustle," said Davis, who said he was high on marijuana and PCP when he shot Stephen Marston, 36, and his wife. "I didn't intentionally kill Mr. Marston. It was an accident and I'm sorry for what I did," he said.

Minutes after Davis testified, Marston's widow, Martha F. Riche, 43, refused to sit in the same chair at the witness stand that Davis had spoken from. Instead, she stood beside it and impassionately explained how much of her life had perished along with her husband that night at 3603 Greenway Place in Parkfairfax.

"I can't stand living without him," she said of her late husband, her voice trembling. "I don't want to live without him."

At the end of the tense two-hour proceeding, Circuit Judge Albert H. Grenadier sentenced Davis to two life terms in prison for capital murder and burglary while armed. Davis also received 20 years for malicious wounding in the shooting of Riche, 12 years for three firearm counts and 30 years for two robbery counts in connection with the shooting.

Grenadier, who characterized Davis' crimes as "unnecessary and senseless," ordered Davis to serve the prison sentences consecutively, which means that he will not be eligible for parole for a least 20 years.

"He could be in his 50s before he could come out," Riche said shortly after the sentencing, as she clung to another woman's arm for support, her eyes red from tears. "That would be the better part of his life. I feel that is justice."

Attorneys who represented Davis said he plans to appeal the sentencing.

Last month, minutes before his trial was to begin, Davis pleaded guilty to capital murder in exchange for an agreement from Commonwealth's Attorney John E. Kloch that the city would not seek the death penalty. He also pleaded guilty to the other seven counts against him in the case.

Davis' brother, Bartel, and Jeffrey D. Harris, both 17, recently pleaded guilty to their involvement in the shootings and robbery. Sentencing is scheduled for next month.

Yesterday, Boggess Davis, who lived with his family at 421 N. Payne St., Alexandria, told Grenadier that the evening of the murder started with an unexpected call from the 17-year-old girl he had been seeing for about two months. She wanted him to take her to the movies, but he had no money, Davis testified.

Davis said he and his brother and Harris piled into a rented car, picked up the girlfriend and on the way they "discussed making some money." Davis said he brought along a loaded pistol. All of them got high smoking marijuana laced with the hallucinogenic drug phencyclidine (PCP), a potent combination called "Lovely," Davis told the judge.

A short time later, after driving through Alexandria, "we saw a man," Davis said. It was Stephen Marston, an economist for the Federal Trade Commission, loading empty boxes from his car in preparation for moving from Parkfairfax near Shirley Highway to the couple's new home in Arlington.

"I went up to the man and I put the gun in his back and told him it was a stickup," Davis said. "He thought I was joking, so I pulled back the hammer on the gun. He pulled his wallet out of his pocket."

"I was ready to go. But Bartel and Jeff ran into the house," he said.

Davis said inside the house someone turned out the lights. Marston "turned and hit the gun with his shoulder and the gun went off," Davis said. He added that he then shot Riche "out of fear" when she rushed toward him.

"Not true," Riche said quietly from her seat in the courtroom.

From the witness stand, Riche said Davis ordered the lights off and almost instantly shot her husband while holding the gun to his back. Davis then shot her in the chest, Riche testified.

"I was standing two to five feet from my husband," Riche said. "My husband didn't move . . . . He fell face down, cried out. His body jerked and I saw my husband die."